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Geomorphology 93 (2

Clay slides in the Målselv valley, northern Norway:Characteristics, occurrence, and triggering mechanisms

Raymond S. Eilertsen a,⁎, Louise Hansen b, Terje H. Bargel b, Inger-Lise Solberg c

a Geological Survey of Norway, Polarmiljøsenteret, N-9296 Tromsø, Norwayb Geological Survey of Norway, N-7491 Trondheim, Norway

c Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway

Received 18 October 2006; received in revised form 22 March 2007; accepted 30 March 2007Available online 19 April 2007

Abstract

The distribution of a large number of clay slides in the Målselv valley, northern Norway, is analysed and put into context withthe stratigraphic organization of the valley-fill sediments. About 32% of the landslides larger than 104 m3 occur close to the valleymargins, where mud is either exposed or at shallow depth. About 57% of the landslides occur mid-valley, where relatively thin(b15 m) coarse-grained deltaic sediments overlay fine-grained marine and glaciomarine sediments, and about 11% of the landslidesoccur in front of ice-contact deposits. The slide-prone areas are all characterized by the occurrence of heterogeneous sediments(interbedded clay, silt and sand), in addition to the presence of steep slopes eroded by rivers. The heterogeneous nature of thesediments probably enhanced groundwater drainage and leaching of salts from the clay, increasing sensitivity. Thus, thedistribution and organization of the valley-fill sediments and groundwater drainage probably controlled the position of the slidescars and sliding planes. Since deglaciation of the valley (11,500 BP–present), isostatic rebound has enhanced fluvial incision andthe creation of steep slopes due to a fall in relative sea level of up to 80 m.

Arcuate-shaped, ‘bottleneck’ landslide scars ranging from c. 104 to 107 m3 in size is the dominant morphological signature ofthe slides, typical for quick clay slides or earth flows involving fluid mud. Their most common triggering mechanism is probablyerosion at the toe slopes by the river Målselv or its tributaries. River erosion close to the valley margin, where glaciomarine andmarine sediments are present, seems to give the most severe slides. The overall valley-fill organization controls the distribution ofclay slides, which may apply to other fjord valleys having similar sediment distribution.© 2007 Elsevier B.V. All rights reserved.

Keywords: Clay slides; Sensitive clay; Slide plane; Valley fill; Northern Norway; Målselv

1. Introduction

The widespread occurrence of glaciomarine and ma-rine clay sediments in Norway has predisposed manyareas to landslides, especially since many contain quickclays (Jørstad, 1968). The term ‘quick clay’ refers to clay

⁎ Corresponding author. Tel.: +47 77750128; fax: +47 77759126.E-mail address: raymond.eilertsen@ngu.no (R.S. Eilertsen).

0169-555X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.geomorph.2007.03.013

whose structure collapses completely on remoulding, andwhose shear strength is thereby reduced to almost zero.Quick clay is defined as clay with a fully remoulded shearstrength of less than 0.4 kPa. The sensitivity is the ratiobetween the undisturbed and the fully remouldedundrained shear strength. The clay is considered to havelow sensitivity if this ratio is b8, medium sensitivity for8 to 30, and high sensitivity for N30 (Rankka et al., 2004).Quick clay slides, hereafter termed clay slides, are a

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serious threat to humans, infrastructure and properties.While traditionally, geotechnical examinations have beenused to evaluate areas susceptible to clay slides inNorway

Fig. 1. Location map showing landslides and surface sediments in the MålsLegend applies to all figures except Fig. 2.

(Gregersen and Løken, 1983), relatively little attentionhas been given to the geological context of the slide areason a larger scale. Specific geological conditions related to

elv valley (modified after Eilertsen, 2002). Contour interval is 20 m.

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the sedimentation of slide-prone material, and post-depositional processes affecting the properties of thematerial (e.g. leaching of pore water salts) and erosion,have had a large impact on the style, timing and cause ofindividual slide events (Viberg, 1984). Understanding thereasons for slide occurrence and development is importantin order to assess future hazards. Furthermore, mappingslides in an area at different spatial scales may provideinsight into the preparatory factors related to theiroccurrence, as well as information for identifyingsusceptible areas (Carson and Geertsema, 2002).

A total of 178 clay slides of varying size and age inthe Målselv valley, northern Norway, have been mappedby Hansen et al. (2002) (Fig. 1). Of these, 54 are largerthan 104 m3. The distribution of slides corresponds withthe distribution of glaciomarine and marine fine-grainedsediments (predominately clay and silt) that make up themajor part (73%) of the total sedimentary fill of theMålselv valley (Fig. 1; Eilertsen, 2002; Eilertsen et al.,2005, 2006). The sediments were deposited in a fjordbasin of variable depth, under relative sea-level fall,during and after the final retreat of the Fennoscandianice sheet. Detailed investigation of a single, well-definedprehistoric landslide in the Målselv valley (Hansenet al., 2007) suggests that local stratigraphy had animportant impact on the dimensions and style of theslide. The distribution of permeable and impermeablelayers in clay was linked to groundwater movement,leading to the development of soft sensitive layers andthe subsequent clay slide.

Eilertsen (2002) and Eilertsen et al. (2005, 2006)studied the stratigraphy and depositional history of thevalley-fill sediments, while Hansen et al. (2002) mappedthe scars of clay slides in the valley. Here we try to linkthese studies and further investigate the clay slideswithin the entire system of the Målselv valley in order to(1) characterize the distribution of the slides in relationto the overall valley-fill stratigraphy, and (2) discuss themost likely causes for the slides.

2. Geomorphological setting

The Målselv valley (Målselvdalen; Fig. 1) is abedrock-confined, glacial-trough valley set in a land-scape of relatively high relief (0 to 1500 m a.s.l). Glacio-isostatic uplift exceeded eustatic sea-level rise duringthe deglacial and postglacial periods, causing emergenceand net relative sea-level fall (Eilertsen et al., 2005).Increase in uplift rates towards the east (closer to theuplift center) has caused the highest and oldest raisedshorelines to tilt seawards by c. 0.8 m km−1. The marinelimit (maximum sea level following ice-margin retreat)

in the valley ranges from c. 60 m above present sea levelin the outer (western) part to c. 80 m in the inner(eastern) part (Eilertsen et al., 2005). The valley floordescends from 80 m a.s.l. in the upper part of the valleyto sea level at the mouth. The major river (Målselva) hasa total catchment area of 6041 km2 (Santha, 1988), andannual snowmelt-flood discharge peaks occasionallyexceeding 1500 m3 s−1. The climate is subarctic, with amean annual precipitation of 940 mm (NorwegianMeteorological Institute, http://www.DNMI.no). Bed-rock in the area consists mainly of Caledonian metamor-phic rocks, typically gneiss, granite, schist and dolomite(Sigmond et al., 1984). Several minor tributary valleyswith small rivers enter the valley.

During the last glacial maximum, the valley wascovered by thick ice, and most of the preglacial valley-fill sediments were eroded and transported to theshelf edge. The deglaciation of the valley commencedaround 11,500 years ago, and was completed around10,300 years ago. The valley has been subject to post-glacial isostatic rebound, with the highest rates of fallingbase level occurring immediately following the deglaci-ation (Eilertsen et al., 2005). The present valley fillaccumulated during two major phases: deglacial andpostglacial (Eilertsen et al., 2006). The deglacial phase ischaracterized by a large sediment supply from the glacierand inundation of the fjord valley during glacier retreat.The ice sheet retreated in a step-wise manner leavingbehind several large and small ice-front accumulations attopographic sills and at the mouths of tributary valleys.Large volumes of fine-grained glaciomarine sedimentaccumulated in basins between the ice-front accumula-tions. The postglacial phase is characterized by an initialrapid fall in relative sea level and high sedimentationrates, followed by amore gradual sea-level fall and lowersedimentation rates. As seen in the longitudinal profile,the valley comprises two major overdeepened basins(termed upper and lower basins; Fig. 2), separated by abedrock sill 35 km south of the present delta rim(Eilertsen et al., 2006). Maximum sediment thickness ismore than 135 m in the upper basin and more than 158 min the lower basin. Of this, up to 125 m consists of fine-grained glaciomarine and marine deposits. Soon afterdeglaciation, fluviodeltaic progradation started, a sce-nario typical of large Norwegian fjord valleys (Corner,2006). Fluvial erosion has, in places, removed large partsof the valley-fill succession creating high-relief slopes.The river incised at an estimated average rate of 0.34 cmyear−1 during the last 7430 years north of Målselvfossen(Eilertsen et al., 2006). Fluvial incision above (south of)the mid-valley sill at Målselvfossen, which determinedlocal base level, occurred at a rate of 0.07 cm year−1

Fig. 2. Depth to glaciomarine and marine sediments (after Eilertsen et al., 2006).

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during the last 5630 years. Five major depositional unitshave been recognized, including glaciofluvial, glacio-marine, marine, fluviodeltaic, and fluvial (Eilertsen et al.,2006). Of particular importance in this study are the

occurrence and distribution of the clay-rich glaciomarineand marine sediments.

Post-depositional landsliding is a natural process inthe valley and influences the redistribution of sediments

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(Corner, 2006). Most of the slides are prehistoric, asevidenced by the elevated position of the slide scars incomparison with present-day rivers, as well as radiocar-bon dates (Hansen et al., 2007). Although quite a fewslides have occurred in historic times, only a few haveproved fatal. One exception is a clay slide at Sandbakken(Fig. 1), which occurred on the 6th of December 1898,causing the death of seven people. In addition, a few slideshave caused severe damage to infrastructure: In May2000, a clay slide destroyed phone and electricity cables atBardufoss, and in June 2001, another clay slide destroyedpart of a road at Mortenelv (Fig. 1). The NorwegianGeotechnical Institute registered 13 quick clay zoneslarger than 1 ha in the valley between Brannmoen andPannesmoen (Fig. 1) based on borehole data and soilsampling (Nerland and Gregersen, 2004).

3. Methods

Hansen et al. (2002) mapped 178 slide scars in thefield. For each slide, area and volume were measured, aswell as other features such as the geometry of the slideplane (if detectable), gradient of slide slope, groundwa-ter outlets, and stratigraphy (see Hansen et al., 2002 fordetails). Karlsrud et al. (1985) concluded that landslidesin Norway larger than 104 m3, involving fine-grainedsediments, are related to highly sensitive clay. Of the178 slides, 54 are larger than 104 m3, and these aretreated in more detail here. Detailed sedimentologicalexamination of field sections within and close to theslide scars was conducted where possible to documentlithology, bedding characteristics and bedding surfaces.Hansen et al. (2002) and Eilertsen (2002) also usedauger drill data and ground-penetrating radar (GPR)imagery to characterize the subsurface sediments of thevalley.

Thirteen sediment samples from four different land-slide sites were collected and analysed by laser diffrac-tion (Coulter) at the Geological Survey of Norwaylaboratory in Trondheim in order to document mineraltypes and grain size.

4. Landslide characteristics

The clay slides in the study area range in size fromsmall (b10 m3) to large (1.8×107 m3). About 70% (124)of the slides are smaller than 104 m3, 23% (41) arebetween 104 and 106 m3, and 7% (13) are larger than106 m3. The landslides (scar and deposits) cover an areaof 4.6×106 m2, or ∼1.7% of the total area of theinvestigated valley below the upper limit of marinesediments. In general, most of the larger slides (N104 m3)

in Målselv are easily recognized based on their arcuate,occasionally bottleneck shape. This morphology istypical for clay slides or earth flows involving fluidmud, where a major part of the sediments becamedisaggregated and escaped during free retrogression(Mitchell and Markell, 1974; Porshmann et al., 1983;Karlsrud et al., 1985). Although rotated intact blocks aresometimes present within a slide scar, much of the spoilwas commonly evacuated from the slide scar, leavingbehind a slightly undulating slide scar floor. The blocksconsist of coarser material which originally covered thefine-grained material. Minor ridges of former terracesediments, oriented normal to the flow, are often presentclose to the back scarp (Fig. 3).

The clays in the landslide deposits consist predom-inantly of quartz, plagioclase, mica (mainly illite), andchlorite. This composition is similar to that reportedfrom other parts of Norway (Gregersen and Løken,1979). Smectite was found in only two of the thirteensamples collected. This is in accordance with Rosenq-vist (1953) and Rankka et al. (2004) who stated that noquick clay contains more than a few percent of swellingminerals of the smectite type. Grain-size analysis of the13 samples indicates a grain-size composition of 12–44% clay, 37–71% silt, and 0–37% sand.

5. Occurrence of landslides

All landslides reported here were initiated withinfine-grained glaciomarine and/or marine sediments. Inthis section, we first present the valley-fill stratigraphyand thereafter discuss the relation between the landslidesand the sediment distribution.

5.1. Valley-fill stratigraphy — distribution of mud

The distribution of glaciomarine and marine sedi-ments within the valley is controlled by the bedrocktopography of the fjord–valley basin. Thickest accumu-lations are found in bedrock depressions, typically mid-valley, while the thinnest, and highest, accumulations arefound at bedrock sills and close to the valley margins. Ina cross-sectional view, the valley floor has a concave,generally asymmetric form that exerted a strong controlon the sediment-infill geometry. The present-day rivercuts into fine-grained sediments in most of the upperbasin and in the upper parts of the lower basin (Fig. 2).However, the river has migrated across the valley floorduring the Holocene reworking and removing much ofthe sediments, resulting in a flat valley floor. Conse-quently, there is little landslide activity in the centralparts of the upper basin. However, high-relief remnants

Fig. 3. Photographs of landslides in the Målselv valley. The slide scars in (a) and (d) are 50 and 60 m across, respectively. Persons for scale in (b)and (c).

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of glaciomarine and marine sediments are present at thevalley margins, in addition to a few isolated deltaicterraces with slopes up to 30 m high in the central part(Fig. 1). A cluster of landslide scars is recorded along theflanks of the deltaic terrace at Kirkesnesmoen. Here, theoverlying coarse-grained deltaic sediment is less than15 m thick, and the rivers cut into underlying marine andglaciomarine sediments (Fig. 2).

Along the valley axes in the northern part of thelower basin, the fine-grained sediments lie more than30 m below the river level, and subsequently there is nopotential for clay slide activity (Fig. 2). On the valleymargin, however, slide-prone sediments are exposed oroverlain by a thin cover of sediments, and this is alsowhere traces of landslides are detected today (Fig. 2).The Målselv river and its tributaries also cut into thesesediments on the western side of the valley.

5.2. Relation between landslides and valley-fillstratigraphy

Almost 32% of the landslides occur close to thevalley margins, where fine-grained sediments are ex-

posed or lie close to the surface. The Mortenelv areaserves as a type area for landslides occurring in thistype of setting (Figs. 1 and 4). Here, numerous land-slides occur along the riverbank of the meandering riverMortenelva, a minor tributary entering the riverMålselva 11 km upstream from the delta rim. Theriver runs parallel to the valley margin for most of itscourse, before turning east near the confluence with theMålselva, dissecting fine-grained glaciomarine/marinesediments and coarse-grained deltaic sediments. Strati-graphic studies in the area have shown that the sedi-ments consist of, in stratigraphic order, till overlain byglaciomarine, marine, and deltaic sediments. The strati-graphic dip is toward the east, and the sediments thickeneastwards (Fig. 4) to more than 100 m, as shown byborehole data (Eilertsen et al., 2006).

A retrogressive clay slide developed during severalstages in the years 2000 and 2001 in the lower reaches ofthe Mortenelva. The slide covered an area of 3500 m2,involving a total sediment volume of 2×104 m3 (Hansenet al., 2002). The slide scar is arcuate-shaped, with a backwall 3 to 4m high (Fig. 5). Although the basal sediment inthe slide is quick clay, the overlying material, forming the

Fig. 4. Details of the Mortenelv locality. (a) Map with landslide locations. (b) Cross-section showing borehole data and inferred stratigraphy. Note theincreased thickness of deltaic sediments towards the east (right). See Fig. 1 for location.

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bulk of the spoil, has low sensitivity, thus preventingliquefaction. Consequently, large intact blocks of rela-tively coarse sediment (silt to gravelly sand) and non-deformed clay are present within the slide scar (Fig. 5).The sediments exposed in the back wall comprise, frombottom to top: (1) laminated silty clay dipping towards theeast, truncated by (2) a 2-m thick unit consisting of troughcross-bedded gravelly sand beds, overlain by massive,

laminated and ripple sand beds (Fig. 5c). The former unitis interpreted as marine sediments deposited throughsuspension settling and by turbidity currents, while thelatter unit is interpreted as fluvial delta topset sediments(Eilertsen et al., 2006). Correlation between borehole andGPR data suggests that the slide plane is located 1–3 mbelow the contact between deltaic topset and underlyingfine-grained marine sediments (Fig. 4).

Fig. 5. Photographs illustrating the 2000–2001 landslide event at Mortenelv. (a) Slide scar in 2000. Soon after the picture was taken the back wallretrogressed 2 m. (b) Slide scar in 2001. Note the upright trees and blocks of coarse-grained sediment still remaining in the scar. The back scarp shownin (a) is to the right. (c) Close-up of a boundary between deltaic and marine sediments. Note the sand beds dipping towards the right (arrows).(d) Picture illustrating the fluid nature of the slide material and complete remoulding of the original sediments.

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About 57% of the landslides occur in the middle partof the valley, most often in connection with high-relief(N10 m) remnants of fluviodeltaic terraces. Oneexample of such an area is Brannmoen (Figs. 1 and 6),where quick clay zones have been mapped (Nerland andGregersen, 2004). Several landslides occur along theflanks of a delta terrace having a relative elevation of c.40 m. Two large slides occur on the southern and easternflanks of the terrace, the largest having removed1.8×107 m3 of sediments. Three smaller landslidesoccur on the western and southern flanks of the terrace,named S1, S2 and S3 respectively, forming typicalarcuate-shaped slide scars. The slides involved a total ofN1.3–3.2×106 m3 of sediments. Radiocarbon dating ofbulk samples of organic material (bog) within thesouthernmost slide scar (S3) gave 3610–3490 14C yearsBP (Hansen et al. 2002) as a minimum age for thelandslide. The basal failure planes in two of these slides(S2 and S3) are located at roughly the same elevation(∼25 m a.s.l.), while the northernmost one (S1) islocated at a slightly lower altitude (∼20 m a.s.l.).Modelling of the boundary between fine-grained marine

sediments and overlying coarser (sandy) fluviodeltaicsediments based on borehole data and ground-penetrat-ing radar profiles shows that the boundary dips towardsthe north (Hansen et al., 2002), explaining the differencein elevation. The sediments, as shown in field sections,consist of tidal rhythmites, and include flat-laminatedsand, silt, and silty clay with thin intercalated silty clayto clay layers, overlain by sandy fluviodeltaic topsetsediments. The tidal influence on the origin of thesefeatures is indicated by the very regular progressivevertical thickening and thinning of individually accretedsets, in response to changing current velocities associ-ated with lunar or lunar–solar tidal cycles (Fig. 6c).

About 11% of the slides occur on the distal side ofice-contact deposits in the Målselvfossen and Bardufossareas (Fig. 1). These deposits formed during retreat ofthe Scandinavian Ice Sheet, when the ice front halted forsome time at the bedrock threshold across the valley atthis location, causing coarse-grained ice-contact sedi-ments to be deposited up to the level of the contempo-raneous sea level. Fine-grained, glaciomarine sedimentsquickly filled the basin distal to the threshold, reaching

Fig. 6. Details of the area around Brannmoen. (a) Map with landslide locations. Seven large and very pronounced slide scars are visible (shown bythick, black lines). (b) Cross-profile with borehole data and inferred stratigraphy. Note the level of the slide floor is closely correlated with theboundary between deltaic and marine sediments. Contour interval is 5 m. (c) Examples of tidal rhythmites within delta bottomset. Note neap–neapcycles (‘N’) separated by winter layers (‘W’). Scale bar in cm. See Fig. 1 for location and Fig. 4 for borehole legend.

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more than 100 m in thickness (Eilertsen et al., 2005).Today, ravines up to 25 m deep dissect the glaciomarinesediments, and slide scars are common. The largestslide scar in the area is located between Årskog andStorleirfallet, where 1.9×106 m3 of sediments wereremoved (Fig. 7). The floor of the slide is located at c.20 m a.s.l. A slide event occurred in 1990, when4×105 m3 of sediment was removed (Figs. 7 and 8d).The spoil overran riprap structures built to protect thearea from river erosion.

Glaciomarine sediments deposited in close proximityto an ice-margin commonly show a wide variety ofgeotechnical properties (Evans, 1982). Examination of

Fig. 7. Details of the area at Brentmoen/Fosshaug. (a) Map showing landslid(b) Cross-profile with borehole data and inferred stratigraphy. See Fig. 1 for loc

several sections in front of the ice-contact depositsrevealed a complex stratigraphy. Coarse-grained sandbeds dipping towards the west are commonly interbeddedwith fine-grained glaciomarine mud. For example, a 7-mthick unit of interbedded sandy ripples and mud laminae,interpreted as submarine channel sediments, occur aboveand below fine-grainedmud (Eilertsen et al., 2005; Figs. 7and 8a–c). The lower boundary of this unit coincides withthe approximate position of the slide plane of a largerlandslide 100 m eastwards. Also, the many ravines in thearea are probably the result of focused groundwaterdrainage and erosion along sandy beds. The presence ofravines prior to the 1990 slide, along with the elongated

e locations. Note the dominance of marine deposits and deep ravines.ation and Fig. 4 for borehole legend (modified fromHansen et al., 2002).

Fig. 8. Photographs from the Brentmoen/Fosshaug area. (a) 30 m high section showing a 7-m thick sandy unit interbedded with clayey sedimentsindicated. (b) Lateral continuity of coarse-grained sand beds within finer-grained glaciomarine sediments. See person for scale. (c) Close-up ofsediments shown in (b). (d) Slide from 1990 at Storleirfallmoen. Note ravines on both sides of the slide scar. Slide width is about 200 and 60 m wide atthe bottom and top, respectively. (e) Back wall of a clay slide at Storleirfallet. Note continuous and discontinuous sand beds between finer sediments.See Fig. 7 for location of the sections.

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shape of the slide scar, suggests that the ravines influencedthe shape of the scar.

6. Discussion

Conditions that lead to slope failures are a complexcombinations of different factors, and it is seldom pos-sible to relate slides to a single triggering factor (Viberg,1984). Below, we discuss the geological conditions thatappear to be prerequisites for the occurrence of clay

slides in Målselv, in addition to the characteristics of thelandslides themselves. Traditionally, clay slide eventshave been ascribed to erosion, large amounts of rainfalland human-induced triggering mechanisms (Gregersen,1981; Evans, 1982). Infiltration of water into mud, re-sulting in increase in pore pressures, and erosion at thetoes of clay slopes, have been considered to be themost important triggering factors for clay slides invarious areas of the world (Bjerrum, 1971; Lafleur andLefebvre, 1980; Lebuis et al., 1983; Viberg, 1984).

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Nearly all of the clay slides in the study area occurredon relatively steep banks bordering the river Målselvaand its tributaries (e.g. Figs. 9 and 10a). Thus, erosion atthe base of the slope seems to be related to the slideevents. For example, the triggering mechanism of theinitial slide at Mortenelva (Fig. 5) is believed to be acombination of heavy rainfalls combined with rivererosion at the base of slope, possibly causing exposure ofquick clay (Solberg, 2003). Heavy rainfall will increasenot only river discharge and river level, but also porepressure (Geertsema et al., 2006). The main landslide atMortenelva probably occurred after the initial smallerslide mass was removed by the river, leaving littlesupport to the sloping sediments behind. However, it isimportant to note that sliding may also occur at mar-ginally stable slopes when the river level is lowered, dueto a reduction in toe load and its support (Tutkaluk et al.,1998; Porter et al., 2002; Clague and Evans, 2003). Also,increased seepage of ground water from the exposedriver bank walls may contribute to erosion and desta-bilization of the river bank.

The distribution of material susceptible to clay slidesfollows the topography of the valley as shown above.Furthermore, the leaching of salts and formation of

Fig. 9. Schematic presentation of valley-fill stratigraphy. (a) Whole valley. (b)the fine-grained sediments close to the valley margin.

sensitive clay are often related to increased or highgroundwater flow, and sand layers within the clay aregenerally known to promote such flow. Bjerrum et al.(1969) argued that the formation of quick clay is mostlydue to leaching caused by an upward flow of freshgroundwater from bedrock protuberances. While thiscertainly may be the case for some areas, many of theareas described here are characterized by quick clayzones many tens of meters above bedrock, suggestingthat some other mechanism must account for their for-mation. For example, the presence of permeable layersthat are connected to surface water or other waterconducting layers greatly enhances the possibility ofleaching (e.g. Figs. 5c and 10b), thereby reducing theliquid limits and shear strength of the clays (Torrance,1987; Lefebvre, 1996).

The back walls of many landslides reported hereconsist of heterogeneous sediments, commonly com-prising sand and silt beds within clay, and occasionallyoverlain by fluvial and deltaic coarse sediments (e.g.Figs. 5c, 6c and 10b). Water will quickly infiltratethrough the coarser sediments, but cannot easily pene-trate the glaciomarine and marine mud below if con-necting layers are absent. Pore pressure in the clay may

Clay slide. Modified from Hansen et al. (2007). Note the distribution of

Fig. 10. Photographs at Storleirfallet and Moen. (a) Erosion at the toe slope of a 30-m high slide-prone section. (b) Pore water drainage throughheterogeneous sediments. Water easily infiltrates the permeable sand layers (arrows), while less permeable mud layers stand out. See Fig. 1 forlocation of sections.

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increase at this contact, triggering failure along claylayers having low shear strength (Clague and Evans,2003). An increase in weight of the overlying coarsesediment due to water saturation by rainfall or snowmeltmay also increase pore pressure in the underlying mud(Hyde and Hunter, 1998). This may be the case for theclay slides occurring at the valley margins and mid-valley, where deltaic sediments overlay glaciomarineand marine sediments (e.g. at Brannmoen and Kirkes-nesmoen, Fig. 1).

The numerous sand and silt beds within the glacio-marine deposits distal to the ice-contact deposit atBrentmoen/Fosshaug (Fig. 1) may promote groundwaterflow. The occurrence of ravines in the same area suggestsgroundwater erosion and drainage along these coarse bedswhich act as aquifers within the fine-grained marine andglaciomarine sediments. Although vertical groundwaterdrainage tends to be impeded, horizontal drainage may beefficient. Also, the stratigraphic dip of the sediments couldfocus groundwater flow, thus increasing pore pressureswhere these aquifers are thin and pinch out (Hansen et al.,2007). The 1990 slide at Storleirfallet (Fig. 8d) is believedto have been caused by pore pressure build-up behind theriprap, when drainage pipes within the riprap were filledand pore fluids could no longer drain freely (Solberg,2003). The abundance of slides close to ice-contactdeposits has also been noticed in southern Norway byBjerrum et al. (1969), and in Canada by Geertsema andSchwab (1997) and Carson and Geertsema (2002).

Aas (1979) found from a study of a number of slidesthat the ‘safe’ height of a ravine in fine-grained materialis about 10 m, and that naturally sloping terrain has to besteeper than 1/15 (3.8°) for a quick clay slide to takeplace. Theoretical analysis later confirmed these values(Gregersen and Løken, 1983). Thus, the ravines and

steep slopes distal to the ice-contact deposit at Fossmoen(Figs. 7 and 8) would qualify as susceptible to failurebased on morphological criteria alone.

Some slide scars appear to have been completelyevacuated, while in other places large parts of the slidematerial remain within the scars (cf. Hansen et al., 2007).The complete disintegration and evacuation of largeamounts of material from many of the slide scarsrecorded in Målselv may suggest that relatively thickquick clay layers were present at the time of the slideevents. However, at localities where most of the spoilremains in the slide scar (e.g. Fig. 3b), only a thin quickclay layer may be present (Karlsrud et al., 1985). The sizeof the quick clay zone may also explain the large sizespan of the landslides. Areas having thick quick clayzones would promote large slides, especially if the slidematerial easily escapes the slide scar. The largest slidesare typically found in areas where sandy delta sedimentsoverlie fine-grained sediments, like at Brannmoen andKirkenesmoen (Fig. 1), suggesting that such areas areparticularly favourable for quick clay formation andsubsequent sliding. The 1990 slide at Storleirfallet isconfined between pre-existing ravines (Figs. 7 and 8)without lateral enlargement. A possible explanation forthis is that the ravines caused ‘drawdown’ of the phreaticsurface along their axes, preventing pore pressure build-up, and hence the sediments could maintain higher shearstrength (Carson and Geertsema, 2002).

Landslides cover only 1.7% of the total valley areabelow the marine limit, suggesting that their contribu-tion to the redistribution of sediments within the valleyis rather small. However, many traces of older slideshave been removed by river erosion, especially in theupper parts of the valley where the river is laterallymigrating to a larger extent than in the lower parts.

561R.S. Eilertsen et al. / Geomorphology 93 (2008) 548–562

The stratigraphic organization in different fjordvalleys in Norway is believed to be similar (Corner,2006), and thus the results presented here may berelevant to other valleys. The main control on generalstratigraphy is basin depth, because it controls accom-modation space. In deep basins, thick fluviodeltaicsediments may cover glaciomarine andmarine clays, andsliding will be constricted to the valley margins, as atMoen and Pannesmoen (Fig. 1). In shallow basins,however, deltaic sediments tend to bypass previouslyfilled portions of the valley, causing rapid delta progra-dation and extensive exposure of fjord-floor clays, oronly a thin cover of delta sediments along the entirewidth of the valley. Brannmoen is an example of thelatter cases, where rapid delta progradation has leftbehind less than 15 m of fluviodeltaic sediments aboveclay.

7. Conclusions

The sliding activity in the Målselv area, northernNorway is related to the valley-fill sediments, inferredfrom lithological, mapping and stratigraphic data. Theresults can be summarized as follows:

• Of the landslides larger than 104 m3, 76% (n=41) aresmaller, and 24% (n=13) are larger than 106 m3. Thelargest recorded slide removed 1.8×107 m3 ofsediments.

• The landslides cover an area of 4.6×106 m2, or∼1.7% of the total area of the valley floor. Due toriver incision and migration, many traces of olderslides may have been removed.

• Due to isostatic rebound and subsequent fall inrelative sea level, rivers have eroded the valley-fillsediments and created a substantial relief (N10 m).

• The presence of relatively steep slopes, river erosionat the toe of slopes, and heterogeneous sedimentswith sensitive clay caused by groundwater drainageand leaching are main prerequisites triggering land-slides in the valley.

• The results presented here are probably relevant toother valleys in northern Norway and possibly else-where with similar sedimentological organization.Understanding the sedimentary architecture of val-ley-fill sediments would be an important contributionto the identification of areas susceptible to landsliding.

Acknowledgements

We are indebted to the inhabitants of Målselv whoprovided access to localities and logistical support. The

comments made by reviewers Oliver Korup and JanKalvoda, along with editor Takashi Oguchi significantlyimproved the manuscript. Geoffrey D. Corner correctedthe English language. This is contribution 143 of theInternational Centre for Geohazards, ICG.

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